Various thermal transfer recording methods have hitherto been known in the art. Among them, a method is proposed to form various full color images by utilizing a dye for sublimation transfer as a recording material, and thermally transferring the sublimation dye from a thermal transfer sheet onto a transfer-receiving material which can be dyed with sublimation dye, wherein the thermal transfer sheet comprises a dye layer formed by holding the sublimation dye by a suitable binder on a substrate such as a polyester film, and wherein the transfer-receiving material includes the thermal transfer image-receiving sheet comprising a dye receiving layer provided on paper, plastic film or the like. In this case, a large number of color dots of three or four colors with the quantity of heat being regulated are transferred by heating by means of a thermal head as heating means in a printer onto the receiving layer in the thermal transfer image-receiving sheet to reproduce a full color of an original by the multicolor dots. In this method, since coloring materials used are dyes, the formed images are very sharp and are highly transparent and thus are excellent in reproduction of intermediate colors and in gradation and are comparable with images formed by conventional offset printing or gravure printing. At the same time, this method can form high-quality images comparable with full-color images formed by photography. Therefore, the sublimation transfer method is used as a means of recording information in various fields.
Due to the development of various hardware and software related to multimedia, the thermal transfer method has been expanding its market as it is utilized for a full color hard copy system for digital images, typical examples of which include computer graphics, still images of satellite communication, CD-ROMs and so on, and analog images such as video. The image-receiving sheets formed by the thermal transfer method have wide-ranging specific usages. Examples of typical usages include printing of proof, output of images, output of planning and design by CAD/CAM, output of various kinds of medical analytical equipment and measuring instruments such as CT scanning and endoscopic cameras, output of photos for identifications, ID cards, credit cards and other cards as alternatives to instant photographs, and output of composite pictures and commemorative photographs at amusement facilities such as amusement parks, game arcades, museums and aquariums.
As one of the important properties required for the thermal transfer sheet of the sublimation transfer method, there is color reproducibility, particularly, reproducibility of black with high density. When forming a thermal transfer image of black by overprinting dyes of three colors from each dye layer of yellow, magenta and cyan in order, there is a problem that such black has poor reproducibility in jet-black color. That is, when dyes of three colors from each dye layer of yellow, magenta and cyan are overprinted in order, reproducibility of jet-black color is difficult, since dye layers of yellow, magenta and cyan need to give priority to balancing reproduction of those colors and intermediate colors, and in addition, there are problems such as subtractive mixture and technical restriction of thermal transfer. Therefore, a dye layer of black itself is required.
As a black thermal transfer sheet having such a dye layer of black itself, it has hitherto been known to obtain a black dye layer by mixing dyes of yellow, magenta and cyan (for example, Patent Literatures 1 and 2).
However, black color tone of printed matter obtained using the conventional black dye layer formed by mixing dyes of yellow, magenta and cyan takes on a red or green tinge, so that desired jet-black color has not always been capable of reproduction. Further, there has been a problem that a gradation difference of black is large.
In addition, printed matter obtained using the black dye layer formed by mixing dyes of yellow, magenta and cyan conventionally has a problem that significant color deterioration occurs due to light. In the case of mixing dyes having different color tone, the phenomenon called catalytic photobleaching, which causes significant color deterioration due to light, is often observed.
Further, in the conventional thermal transfer sheet having the black dye layer, the dye is transferred onto the heat-resistant slip layer provided on the back side of the thermal transfer sheet during storage in a wound state, and, at the time of rewinding, the dyes transferred onto the heat-resistant slip layer are retransferred (kicked back) onto dye layers of other colors or the like. If the contaminated dye layer is thermally transferred onto a thermal image-receiving sheet, the color is deviated from the designated color or a phenomenon, so-called “scumming” is caused.
On the other hand, Patent Literature 3 discloses a thermosensitive transfer sheet provided with color material layers of yellow, magenta and cyan, wherein each of the color material layer contains a thermal transferable pigment and a binder, and mentions a pigment used in the present invention as an example of a thermal transferable pigment contained in the color material layer of each color. However, Patent Literature 3 discloses a thermal transfer sheet wherein a black thermal transfer image is formed by overprinting dyes of three colors from each dye layer of yellow, magenta and cyan in order, and there is no description of black color tone. As shown in Comparative example described hereinafter, there is a problem that the combination of dyes disclosed in Patent Literature 3 cannot reproduce jet-black color desired as black color tone, and has large gradation difference of black so that smooth gradation of black is hardly realized.